Method of improving the magnetic properties of cobalt substituted magnetite

ABSTRACT

A method of improving the magnetic properties of cobalt substituted magnetite by magnetizing the material to the saturation level in the desired direction, and then removing the magnetizing field. The material will retain a level of magnetization normally referred to as the remanent state of magnetization or simply remanence. The magnetized material is then subjected to a heat treatment to anneal the material. The above process significantly improves coercivity, hysteresis loop squareness ratio, and resistance to remanence loss due to external forces.

United States. Patent 1191 Chen et a1. 1 1 Jan. 7, 1975 [5 METHOD OFIMPROVING THE MAGNETIC 3,546,675 12/1970 Che Chung-Chow ct 111.. 117/235x OP S O COBALT SUBSTITUTED 3,573,980 4/1971 Haller etkal. 117/238.620,841 11/1971 Comstoc et a1 117/235 X MAGNETITE 3,703,411 11/1972Mclezoglu 117/235 X [75] Inventors: Shih-Lu Chen; James A. Murphy,5.725.126 4/1973 Hallcr c1111. 117/235 both of Painted Post, N.Y.

Primar E.\'aminer-Williz1m D. Martin 73 A. c G1 k Y ss1gnee Nogiimg assWor 5, Coming Assistant E.mminerBcrnard D. Pianalto Attorney, Agent, orFirmWalter S. Zcbrowski; Flledi y 1972 Clarence R. Patty, Jr. [21] Appl.No.: 257,335

[57] ABSTRACT [52] 11.8. C1 117/237, 117/235, 117/238, method ofimproving the magnetic properties of co- 148/108 balt substitutedmagnetite by magnetizing the material [51] Int. Cl. 1101f 10/00 to thesaturation level in the desired direction, and [58] Field of Search 117/234-240; then removing the magnetizing field. The material will I148/108 retain a level of magnetization normally referred to as theremanent state of magnetization or simply rema- [56] References Citednence. The magnetized material is then subjected to a UMTED STATESPATENTS heat treatment to anneal the material. The above pro 2 919 20712/1959 Scholzel 117/236 cess significantly improves coercivity,hysteresis loop 3:047:423 7/1962 Eggenberge r etaljziiliiiii: 117/238Squareness ratio and resistance remanence loss due 3,092,510 6/1963Edelman 117/238 exter forces- 3,374,113 3/1968 Chang et a1. 117/238 UX3.393.982 7/1968 Fisher et a1. 117/23811x 12 10 Drawmg Fgures e 0065MAX/CM H (OERSTEDS) APPLIED FIELD 9 (GAUSS-CENTIMETERS) I PATENTEDJAN71915 3.859.129

SHEET UlUF1O E5 3 5' 0 ,Q) l- Q g a: .9, E

O '(sazuawlmaa ssnvs) e PATENTEDJAN 71915 R 3,859,129

sum near 10 COERCIVITY 0.8 -5o0 g 9 :2 *5 co 0: 0 0:

0.7 400 g w v' 0') Lil g HYSTERSIS LOOP g SQUARENESS RATIO w 0.5 zoo oCOBALT to IRON RATIO (Co/ Fe) (PRIOR ART),

PATENTEU 3,859,129

' sum 0m 10 Co/ Fe 0.|2

'0, (PARALLEL) H (PERPEN- DICULAR) HC(PARALLEL) IO H (OERSTEDS) PARALLEL100p e =009|5 MAX/CM Gr/G =O.682 H 490 Oe PERPENDICULAR loo p e 0.0885MAX/CM Fig. 4

COERCIVITY (OERSTEDS) PATENTEDJA" 71% 0.5 COBALT TO IRON RATIO (Co/Fe)COERCIVITY (OERSTEIDS) PMENTEUJAH 7:975 3,859 129 SHEET 0am 10' 0 OJ 0.20.3 0.4 0.5 COBALT T0 IRON RATIO (Co Fe) Fig. 8

PATENTEDJAN H915 SQUARENES RATIO (e /9, ,0 4 01 m SHEET near 10UNTREATED FILM (PRIOR ART) COBALT TO IRON RATIO (Co Fe) Fig. 9

PAIENIEB M 3.859.129

, i l 2 PERCENTAGE OF REMANENCE l m SHEET IUUF 10 UNANNEALED FILM (PRIORART) Fig. /0

BACKGROUND OF THE INVENTION I. Field of the Invention This inventionrelates to a method of improving the magnetic properties of cobaltsubstituted magnetite by inducing uniaxial anisotropy in the material inaddition to the already present cubic anisotropy. Magnetic and recordingstorage devices having a film of cobalt substituted magnetite may beused for storing digital information used by data processing computers,or any other analog or digital information where magnetic storage isdesired.

II. Description of the Prior Art It is known by those skilled in the artthat heating cobalt substituted magnetite material to an elevatedtemperature less than the Curie temperature of the material while thematerial is being subjected to a magnetic field will result in auniaxial anisotropy being induced in the material in the direction ofthe magnetic field. The magnetic field applied during heating of thematerial does not directly induce the resulting uniaxial anisotropy ofthe material, but merely aligns the magnetization of each crystal oreach domain in substantially the same direction. It is understood thatwhen the material is subjected to an elevated temperature less than theCurie temperature, a uniaxial anisotropy is induced in each crystal of apolycrystalline material or each domain of a single crystal whether ornot a magnetic field is applied. If the magnetic field is not applied,the uniaxial anisotropy of each crystal or domain is induced in a randomdirection. The uniaxial anisotropy superimposes itself on the normalcubic anisotropy. Additional information on this phenomena is availablein the following publications: Physical Review, Volume 99, page 1788,1955, by R. M. Bozorth, E. F. Tilden, and A. J. Williams; Proceedings ofthe Institute of Electrical En gineering, London, Volume 104, Part BSupplement 7, page 412, 1957, by Wijn, Van der Heide, and Fast; Journalof the Physical Society of Japan, Volume 13, page 58, 1958, by ShuichiLida, I-Iisashi Sekizawa, and Yoshimichi Siyama; Physical Review, Volume108, page 271, 1957, by R. F. Penoyer and L. R. Bickford, Jr.; andJournal of Applied Physics, Volume 29, page 441, 1958, by L. R.Bickford, Jr., J. M. Brownlaw and R. F. Penoyer.

Cobalt substituted magnetite materials have previously been used in thefabrication of some types of magnetic devices such as permanent magnets.The magnetic qualities of such devices have been improved by heatingthem to an elevated temperature below the Curie temperature while theywere subjected to a magnetic field. I-Iowever,.using such a technique toimprove the magnetic qualities of magnetic recording and storage devicesis made very difficult, if not impossible, by practical considerationssince magnetic recording and storage devices are ordinarily in the shapeof a tape, drum, rod or disk. To improve the magnetic properties of adrum shaped device, for example, the uniaxial anisotropy must beoriented circumferentially around the drum axis along the cylindricalwalls, and to improve the magnetic properties of a disk shaped devicethe uniaxial anisotropy must be oriented circularly around the axis onthe disk surface. Providing magnetic fields to achieve the desiredorientation of the uniaxial anisotropy of such devices while they arebeing heat treated presents very difficult practical problems.

SUMMARY OF THE INVENTION It is therefore, an object of this invention toprovide a method of inducing uniaxial anisotropy having a desiredorientation in a cobalt substituted magnetite material.

It is a further object of this invention to provide a simple andeconomical method ofinducing uniaxial anisotropy having a desiredorientation in a magnetic recording and storage device employing cobaltsubstituted magnetite material as the storage medium, which methodovercomes the heretofore noted disadvantages.

Briefly, according to this invention magnetic properties such ascoercivity, hysteresis loop squareness ratio, and resistance to loss ofremanence due to abrasion and other external forces may be significantlyimproved in magnetic devices having cobalt substituted magnetite as themagnetic material by magnetizing the material to the saturation level ina desired direction, and then removing the magnetizing field. Thematerial will retain a level of magnetization normally referred to asremanence. The magnetized material is then subjected to a heat treatmentto anneal the material.

Additional objects, features and advantages of the present inventionwill become apparent to those skilled in the art from the followingdetailed description and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the majorhysteresis loop of a device having a cobalt substituted film, which hasnot been improved by the method of the present invention.

FIG. 2 is a diagram showing the effect of variations in the cobalt toiron ratio upon the coercivity and the hysteresis loop squareness ratioof a magnetic device having an unimproved cobalt substituted magnetitefilm.

FIGS. 3-6 are diagrams showing the major parallel and perpendicularhysteresis loops of four magnetic devices having cobalt substitutedmagnetite films improved by the method of the present invention.

FIGS. 7 and 8 are diagrams showing the relationship between coercivityand the cobalt to iron ratio of magnetic devices having cobaltsubstituted films.

FIG. 9 is a diagram showing the relationship between the hysteresis loopsquareness ratio and the cobalt to iron ratio of magnetic devices havingcobalt substituted magnetite films.

FIG. 10 is a diagram showing' the relationship between the loss ofremanence in cobalt substituted magnetite films due to external forces,and the cobalt to iron ratio.

DETAILED DESCRIPTION A magnetic recording and storage device is formedhaving a non-magnetic substrate to which a magnetic film containingcobalt substituted magnetite is bonded. As used herein, the term cobaltsubstituted magnetite means a magnetic material having a cubiccrystalline structure, similar to the crystalline structure of magnetite(Fe 0,) in which cobalt, iron and oxygen are combined according to theformula Co,Fe ,O The magnetic properties of cobalt substituted magnetitemay be improved by treating the material according to the method of thepresent invention for amounts of co- No. 3,015,628 entitledFerroso-Ferric Oxide for Magnetic Impulse Record Members" issued toJoseph W. Ayers and Robert A. Stephens; US. Pat. No. 3.047.505 entitledMagnetic Recording Media" issued toArthur theainount of cobalt inmagnetite'by referring to the 5 Miller; and U.S. Pat. No. 3,330,693entitled Method value of x, such amount is herein indicated by the coofMaking a Magnetic Record Member with Encapsubalt to iron ratio, orsimply Co/Fe. The cobalt to iron lated Ferromagnetic Particles in aBinder and Resulting ratio is related to x by the equation Co/Fe =x/3-x.For Product issued to George C. Rumberger. Techniques example, where xequals 0.1, Co/Fe 0.l/30.l or for fabricating devices having a solidnon-particulate 0.0345; and where x equals 1.2, Co/Fe= l.2/3l.2 ormagnetic film chemically bonded to a substrate, al- 0.665. Althoughfilms having a Co/Fe between a low though available, are not as wellknown. However, invalue of ab0ut 0.()345 a d a hi h value f b t 0,665formation concerning such techniques may be found in may be improved bythe method of this invention, the 2i9l9,207 entitled Method of pp y mostsignificant improvement occurs in films having a a Ferromagnetic surfaceto a Base Utilizing Iron C Co/Fe of between about 0.1 and 0,25 15 bonyland oxygen issued to Karl Schol'zel, and in the The substrate or supportmember may have any suittwo P e applicfltlohs entitled f of Making aable form such as a tape, disk, rod, drum or the like, Magnet":keciordlhg and storage'Devlce by James and may be formed from anysuitable non-magnetic P Y Halaby and Neal Kenny material that canwithstand the temperatures encoun- 1511356 1795542 and tered in themethod of this invention without damage. 151,388 both filed 1971'Examples of such materials are vinyls, plastics, anod- A methoq Pmducmga cobalt subsmuted magne' ized aluminum, glass-ceramic,,glass, ceramicand the film chenllcanyfbonded to the .substhate like. The presentinvention is not limited to any specific w Pamcularly Stumble or thepresent mvemlon substrate material. For magnetic devices not requiringas 0 a highly flexible substrate, a particularly suitable mate- Thevapors of an iron containing compound that will rial for forming a disk,rod, drum or the like is ionnot decompose when vaporizedmnd the p s aexchange strengthened glass or glass-ceramic. A basic balt containingcompound that will not decompose discussion of ion-exchangestrengthening processes is When v p z are transported o a a ed u stratefound in Stresses in Glass Prod ced b N JJ if where the vapors areallowed to react. Such iron and Exchange of Monovalent Ions by S. F.Kistler, pubcobalt containing compounds include but are not limlished inthe Journal of the American Ceramic Society, ited to those Show" inTable The Vapors 0f Ofthe February, 1962, pages 59-68, iron containingcompounds listed in Table I may be sat- The magnetic film containingcobalt substituted maglsfactorhy used with the Vapors of any of thecobalt netite may be a solid non-particulate film chemically containingcompounds listed in Table I.

Table I Cobalt Containing Materials Iron Containing Materials cobaltnitrosyltricarbonyl iron pentacarbony1-Fe(CO) 5 Co( O)3NO cobaltocene-Co(05115) 2 terrocene-Fe (0 115) 2 cobaltic acetylacetonate eobaltousacetylacetonate cobaltie hexafluoroacetylacetonate cobaltoushexafluoroacetylaeetonate bonded to thesubstrate, or maybe particles ofthe cobalt substituted magnetite material bonded to each other and tothe substrate by binder or filler materials such as epoxies, urethanes,vinyls, and the like. Techferric acetylacetonate ferrous acetylacetonateferric hexafluoroacetylacetonate ferrous hexafluoroaeetylacetonate Inaddition, the iron containing compound ferric chloride (FeCl maysatisfactorily be used with the cobalt containing compound cobaltchloride (CoCl Depending upon the cobalt containing compound and ironcontaining compound used, the temperature of the substrate upon whichsaid vapors are allowed to react may be varied from about 200C to anupper limit determined by structural limitations of the substrate.However, in most cases the desirable range will be from about 350C toabout 550C. The film resulting from the reaction of the vapors on theheated substrate is a non-magnetic cobalt-iron oxide. The non-magneticcobalt-iron oxide film is then reduced to a magnetic cobalt substitutedmagnetite material by subjecting the substrate and non-magneticcobalt-iron oxide film to a suitable reducing atmosphere, such as ahydrogen and water atmosphere. ,The reduction is accomplished by heatingthe film and substrate combination to a temperature of between 300C anda maximum temperature determinedby structural limitations of thesubstrate while maintaining the combination in a reducing atmosphere.

Atmospheres particularly suitable for use in this invention include butare not limited to a hydrogen and water mixture, a carbon monoxide andcarbon dioxide mixture, and a carbon monoxide and water mixture. Aninert gas, such as nitrogen, maybe combined with these reducingatmospheres without significantly reducing the effectiveness thereof. Anatmosphere of hydrogen and water in combination with nitrogen, which isparticularly suitable for use with the method of this invention, may beobtained by bubbling a mixture of hydrogen and nitrogen through water.The important consideration of this particular atmosphere is thehydrogen partial pressure to water partial pressure ratio. The nitrogen.is substantially inert and acts only as a carrier gas for the water sothat the ratio of hydrogen to water in the system is more easilycontrolled. The allowable range of hydrogen partial pressure to waterpartial pressure ratio which will produce the desired atmosphere forconverting the non-magnetic cobalt-iron oxide will varyas thetemperature of the film and substrate combination varies. A hydrogenpartial pressure to water partial pressure ratio range of between :1 and":1 may be used for temperatures between about 300C and 600C. Atmospheretemperatures greater than 600C may be used if the ratio is maintainedbetween about l0:l and l0*:l. After reduction is completed, thetemperature of the film and substrate should be reduced to lessthanabout 200C in the shortest possible time to achieve the highestimprovement as a result of the present method.

g The magnetic device as described is subjected to a magnetic field tomagnetize the film in the desired di rection to substantially thesaturation level of the material. The magnetic field strength requiredto magnetize the device to the saturation level will depend upon themagnetic properties of the device. For example, FIG. 1 shows the majorhysteresis loop of an untreated cobalt substituted magnetite film havinga cobalt to iron ratio of about 0.21. Symbol 9 represents the magneticflux in the material times the film thickness, hereinafter referred tosimply as magnetization, which magnetization is brought about by theapplied field. In FIG. 1, magnetization 9 is in gauss centimeters andthe applied field H is in oersteds. From FIG. 1, it can be seen that themaximum remanence is obtained with an applied magnetic field of about 1100 oersteds since that is the field necessary to magnetize the film tosubstantially saturation. As can also be seen from FIG. 1, the coercivity H of such a film is approximately 370 oersteds. As heretoforenoted, the field necessary for saturation will vary depending upon themagnetic properties of the device, however, as a practical matter anexternal field of between 4000-7500 oersteds may be satisfactorily usedfor many of the described devices since an excessive field will do noharm whereas an insufficient field will result in an unnecessarily lowremanence and, consequently, the improvement as a result of the processof this invention will not be as great as possible. After the externalmagnetic field has been removed the remanent magnetization in the devicealigns the uniaxial anisotropy in the cobalt substituted magnetite filmin the same direction thereby eliminating the need for an external fieldto be applied while the device is undergoing the extreme temperaturesencountered during annealing.

ln employing the method of the present invention. the device can bemagnetized in the desired direction a portion at a time by any suitablemeans. including the apparatus normally used to record on the magneticdevice. For example, magnetic disks may be magnetized substantially tosaturation by applying a continuous DC signal to the recording headwhile rotating the disk in the normal manner in relation to the head.More specifically, the head may be positioned over one track and thedisk rotated a full revolution thereunder in the normal manner. The headwould then be repositioned over an adjacent track and the disk againrotated a full revolution. This procedure would be continued until allof the tracks of the disk are completely magnetized in the desireddirection. Magnetic recording and storage devices having other shapescould similarly be magnetized by the device utilization means.Obviously, equipment specially built for this purpose can also be used.

After the desired remanence and alignment is obtained, the device isheat treated to anneal the cobalt substituted magnetite film. Such heattreatment may be satisfactorily carried out in air, but any atmospherenot reactive with the cobalt substituted magnetite or with the substratematerial at the annealing temperature may be used. The time necessaryfor proper annealing varies with temperature and the cobalt to ironratio. High temperatures require less annealing time and lowtemperatures require more annealing time. However, it has been foundthat satisfactory results may be obtained even when the device isannealed for a period of time well in excess of that which is necessary.Good results have been obtained when time period ranged from two hoursto 177 hours for temperatures between about 200C and C respectively. Ifthe temperature is below about 95C, annealing is so slow that the totalrequired time is excessive and therefore unsatisfactory. On the otherhand, if the temperature is above about 200C, process control becomesmore difficult, and the results become less predictable since the cobaltsubstituted magnetite begins to becomechemically unstable and mayconvert to a nonmagnetic oxide. It has been found that excellent andconsistent results can be obtained by using a temperature of betweenabout C and C for a time period of about 147 hours.

As heretofore noted, the cobalt to iron ratio of cobalt substitutedmagnetite directly affects some of the magnetic characteristics of thematerial. Referring to FIG. 2, it is seen that the coercivity of acobalt substituted magnetite film increases and thehysteresis loopsquareness ratio decreases as the cobalt to iron ratio increases. Thereasons for these changes are not clear. It is believed, however, thatthe cobalt to iron ratio in the ferromagnetic phase of the film is lessthan the overall ratio, and that excess cobalt may exist as a secondnon-ferromagnetic oxide phase or in clusters of paramagnetic material.The presence of either non-ferromagnetic oxide or paramagnetic cobaltclusters would reduce the squareness ratio. If the cobalt substitutedmagnetite film was reduced in a reduction atmosphere of hydrogen andwater as heretofore described, it is believed, based upon the fact thatmetallic cobalt is more stable than cobalt oxide under such reductionconditions, that cobalt clusters are likely to be the cause of the lowsquareness ratio.

The method of this invention will improve both the coercivity andhysteresis loop squareness ratio of cobalt substituted magnetite. Theterm hysteresis loop squareness ratio when used herein means remanence,0,, divided by the magnetization occuring when a field of 10,000oersteds is applied, 9 That is, hysteresis loop squareness ratio G /GFIGS. 3, 4, and 6 show the hysteresis loops of four samples havingdifferent cobalt to iron ratios after a 147 hour annealing at about 150Cin air. Prior to the annealing, these samples were magnetized tosubstantially the saturation level in a field of 7500 oersteds. FIGS. 3,4, 5 and 6 each contain two hysteresis loops. One of the hysteresisloops results when the present device is subjected to a field parallelto the magnetization field which was applied before annealing, and thesecond loop results when the device is subjected to a fieldperpendicular to the magnetization field applied before annealing.Hereinafter, the hysteresis loop resulting from parallel magnetizationand the hysteresis loop resulting from perpendicular magnetization willbe referred to as the parallel and perpendicular loops respectively. Theparallel and perpendicular loops are quite different except in FIG. 3,where they almost coincide with each other. The parallel loop has a muchhigher squareness ratio and coercivity than does the perpendicular loop.v

FIG. 1 illustrates the hysteresis loop of an untreated cobaltsubstituted magnetite film having a cobalt to iron ratio of 0.21. FIG. 5illustrates the hysteresis loop of a'treated cobalt substitutedmagnetite film having a ratio of 0.19. Although the cobalt to ironratios are not identical, they are close enough to illustrate theimprovement in the coercivity and hysteresis loop squareness ratiowhichresults when the magnetic devices are treated in accordance with thepresent invention. The cobalt to iron ratio in the untreated filmillustrated by FIG. 1 is higher than the cobalt to iron ratio of thetreated film illustrated by FIG. 5, yet the film illustrated by FIG. 5has significantly higher coercivity. In addition, the parallel andperpendicular hysteresis loops have a significantly higher squarenessratio than does the hysteresis loop of the untreated film.

Graphs which illustrate the coercivity and squareness ratio for somefilms annealed under different conditions are shown in FIGS. 7, 8 and 9.FIG. 7 illustrates the effect of the cobalt to iron ratio on thecoercivity of films annealed at 150C according to the method of thisinvention for different periods of time. FIG. 8 illustrates the effectof the cobalt to iron ratio on the coercivity of films annealed atdifferent temperatures and for different periods of time. FIG. 9illustrates the effect of the cobalt to iron ratio on the hysteresisloop squareness ratio of films annealed at 150C for different periods oftime. FIG. 10 illustrates the effect of the cobalt to iron ratio on theamount ofremanence loss due to abrasion and other external forces uponfilms annealed at about 150C for different periods of time. The contentof FIGS. 7 through 10 are summarized in Table II.

Table Il-Continued Annealing Magnetization Figure Curve Temp. C TimeHrs. Direction 7 E 150 I8 Perpendicular 8 F 200 2 Parallel 8 r G 200 2Perpendicular 8 H 177 Parallel 8 l 95 I77 Perpendicular 9 .l 147Parallel I 9 K 150 18 Parallel 9 L 150 I47 Perpendicular 10 M 150 18Perpendicular l0 N 150 82 Perpendicular l0 0 Not Magnetized 10 P 150 18Parallel 10 Q 150 82 Parallel The effects of these different annealingconditions can be summarized as follows: (A) The coercivity and thesquareness ratio for both the parallel and perpendicular loops of anannealed film are higher than those for a non-annealed sample with acomparable cobalt to iron ratio. (B) Films annealed at high temperatureshave higher coercivity than those annealed at lower temperatures; (C)The coercivity of the material is significantly higher when magnetizedparallel to the initial magnetic field than when the material ismagnetized perpendicular to the initial magnetic field. (D) Filmsannealed at lower temperatures seem to have a slightly higher squarenessratio than those annealed at higher temperatures.

The improvement of the coercivity and hysteresis loop squareness ratioas a result of the present treatment may be explained if the likelypossibility that the cobalt substituted magnetite film was notmicroscopically homogeneous is assumed. If such nonhomogeneity existed,it may further be assumed that a homogenization process went on duringthe heat treatment, and that some of the paramagnetic cobalt clustersdiffused to formeither ferromagnetic oxide or both ferromagnetic oxideand non-magnetic iron oxide. Both the normal cubic anisotropic constantand the uniaxial anisotropic constant could increase as a result of sucha homogenization process and the increase in ferromagnetic oxide. Adecrease in anumber of paramagnetic clusters and an increase in thecubic and uniaxial anisotropic constants would increase both thehysteresis loop squareness ratio and the coercivity of the film.

A third and very important improvement of the magnetic properties of atreated cobalt substituted magnetite film is that the tendency to loseremanence due to abrasion or to other types of external forces isgreatly reduced. The tendency to lose remanence due to external forcesappears to increase as the cobalt to iron ratio in the film increases,and such increase is probably due to the increase of themagnetostriction constant. However, after the magnetic properties of afilm have been improved by the method of the present invention, thetendency to lose remanence due to externally applied forces becomeshighly dependent upon the direction of magnetization with respect to theinduced uniaxial anisotropy. For example, when a film is magnetized inthe same direction'as the induced uniaxial anisotropy, the remanenceloss due to external forces is much less than when the treated film ismagnetized perpendicular to the direction of the induced uniaxialanisotropy. If the film was annealed without being initially magnetized,the tendency to lose remanence will be the same no matter in whatdirection the film is subsequently magnetized, and this loss will beintermediate to that of losses when the film is initially magnetized ineither the parallel and perpendicular directions. The results of severalsamples tested for remanence losses are shown in FIG. 10. The values 9,and fi represent the remanence before and after external forces wereapplied respectively. Therefore, the percentage of remanence loss isequal to l(6 -6 )/6 Even though cobalt substituted magnetite materialhas a high magnetostriction constant, the high loss of remanenceoccuring in films magnetized perpendicular to the initial magnetizationis in excess of what would be expected if such losses depended only uponthe magnetostriction constant. Therefore, it is believed that theexcessive loss of remanence of a cobalt substituted magnetite film maybe due to the possibility that the effect of an applied force upon amagnetic material depends not only upon the inherent properties of thematerial, such as the magnetostriction constant and the anisotropicconstant, but also upon the physical state of the material itself. Aportion of a single crystal will react to an external force quitedifferently than will a piece of polycrystalline material or a thinfilm. The difference arises from mechanical interaction betweenneighboring particles and between particles and the substrate. A singlecrystal can change its shape or dimension without producing internalstrain. In a polycrystalline material or thin film, however, thesituation is quite different. Since the crystals are tightly bound tothe substrate and tightly packed against each other, they can no longerchange their shape and dimensions freely. Therefore, when a film iscooled down from the high temperature at which it was formed, themagnetization in each crystal will be oriented in a direction such thatthe total energy of the film as a whole will be minimum. This totalenergy includes the crystalline anisotropic energy, magnetostrictionenergy, and elastic energy. When the film is magnetized, an internalstrain will be created which will increase the total energy of the filmbecause the crystals are not free to change their dimensions. This typeof internal strain, although negligible in some material, is notnegligible in cobalt substituted magnetite, and is believed to beresponsible for the high loss in remanence when an external force isapplied to a non-annealed sample. However, if the magnetized film isannealed, some of the internal strain is believed to be relieved.Consequently, when the film is subsequently magnetized in the samedirection as the initial magneti zation, internal strain is much lessthan it would be if the film was subsequently magnetized in a directionperpendicular to the initial magnetization. The difference in internalstrain is believed to explain why the loss in remanence varies sogreatly with the direction of magnetization.

Specific Example until an approximately 3000 A thickness film ofnonmagnetic cobaltiron oxide was deposited thereon. The film was thenreduced to cobalt substituted magnetite by being subjected to a waterand hydrogen atmosphere at 450C for 1% hours. An atmosphere having ahydrogen partial pressure to water partial pressure ratio of about 2.421was used for reducing the film. The atmosphere was obtained by bubblinga mixture of 8% by volume of hydrogen and 92% by volume of nitrogenthrough water, while said hydrogen, nitrogen and water were maintainedat approximately 25 C. The magnetic device was then magnetized to thesaturation level with a circumferential orientation. The magnetic devicewas then annealed in air at 150C for M7 hours. FIG. 5 illustrates theparallel and perpendicular hysteresis loops obtained from the device ofthis example.

Although the present invention has been described with respect tospecific details of certain embodiments thereof, it is not intended thatsuch specific details be limitations upon the scope of the inventionexcept insofar as is set forth in the following claims.

We claim:

1. A method of improving magnetic properties of a magnetic devicecomprising the steps of forming a cobalt substituted magnetite filmhaving a cobalt to iron ratio of between about 0.0345 and about 0.665,said film being bonded to a nonmagnetic substrate, thereafter subjectingsaid cobalt substituted magnetite film to a magnetic field havingsufficient field strength and an orientation to magnetize said film in adesired direction,

removing said film from said magnetic field, said film retaining aremanent level of magnetization in said desired direction, andthereafter annealing said magnetized film at a temperature of betweenabout 95 C and about 200C for at least about 2 hours.

2. The method of claim 1 wherein said cobalt substituted magnetite filmhas a cobalt to iron ratio of between about 0.1 and about 0.25.

3. The method of claim 1 wherein said magnetic field strength issufficient to magnetize said material to the saturation level. A

4. The method of claim 1 wherein said annealing is performed at atemperature between about C and about C.

5. The method of claim 1 wherein said film is annealed for a period oftime between about two hours and about 177 hours.

6. The method of claim 1 wherein said annealing is performed in air.

7. The method of claim 1 wherein said cobalt substituted magnetite filmhas a cobalt to iron ratio of between about 0.1 and about 0.25, saidmagnetic field strength is sufficient to magnetize said film to the sawration level, and said annealing is performed at a temperature betweenabout 150C and about 160C for a period of time between about 2 hours andabout 147 hours.

8. The method of claim 1 wherein said non-magnetic substrate is formedof material selected from the group consisting of anodized aluminum,glass-ceramic, glass, and ceramic.

9. The method of claim 1 wherein forming a cobalt substituted magnetitefilm comprises the steps of heating said substrate to a temperature ofbetween about 350C and about 550C, exposing a surface of said heatedsubstrate to vapors of a cobalt containing compound and simultaneouslyto vapors of an iron containing compound, to form a film of non-magneticcobalt-iron oxide chemically bonded to said substrate, and thereafterheating said film of non-magnetic cobalt-iron oxide to a temperature ofbetween about 300C and about 600C in a reducing atmosphere to reduce thematerialof said film to cobalt substituted magnetite. 10. The method ofclaim 9 wherein said cobalt containing compound is selected from thegroup consisting of cobalt nitrosyltricarbonyl, cobaltocene, cobalticacetylacetonate, cobaltous acetylacetonate, cobaltichexafluoroacetylacetonate, and cobaltous hexafluoroac'etylacetonate, andsaid iron containing compound is selected from the group consisting ofiron pentacarbonyl, ferrocene, ferric acetylacetonate, ferrousacetylacetonate, ferric hexafluoroacetylacetonate, and ferroushexafluoroacetylacetonate.

11. The method of claim 9 wherein said cobalt containing compound iscobalt chloride, and the iron containing compound is ferric chloride.

12. The method of claim 9 wherein said nonmagnetic substrate is made ofan ion-exchange strengthened glass.

1. A method of improving magnetic properties of a magnetic devicecomprising the steps of forming a cobalt substituted magnetite filmhaving a cobalt to iron ratio of between about 0.0345 and about 0.665,said film being bonded to a non-magnetic substrate, thereaftersubjecting said cobalt substituted magnetite film to a magnetic fieldhaving sufficient field strength and an orientation to magnetize saidfilm in a desired direction, removing said film from said magneticfield, said film retaining a remanent level of magnetization in saiddesired direction, and thereafter annealing said magnetized film at atemperature of between about 95* C and about 200*C for at least about 2hours.
 2. The method of claim 1 wherein said cobalt substitutedmagnetite film has a cobalt to iron ratio of between about 0.1 and about0.25.
 3. The method of claim 1 wherein said magnetic field strength issufficient to magnetize said material to the saturation level.
 4. Themethod of claim 1 wherein said annealing is performed at a temperaturebetween about 150*C and about 160*C.
 5. The method of claim 1 whereinsaid film is annealed for a period of time between about two hours andabout 177 hours.
 6. The method of claim 1 wherein said annealing isperformed in air.
 7. The method of claim 1 wherein said cobaltsubstituted magnetite film has a cobalt to iron ratio of between about0.1 and about 0.25, said magnetic field strength is sufficient tomagnetize said film to the saturation level, and said annealing isperformed at a temperature between about 150*C and about 160*C for aperiod of time between about 2 hours and about 147 hours.
 8. The methodof claim 1 wherein said non-magnetic substrate is formed of materialselected from the group consisting of anodized aluminum, glass-ceramic,glass, and ceramic.
 9. The method of claim 1 wherein forming a cobaltsubstituted magnetite film comprises the steps of heating said substrateto a temperature of between about 350*C and about 550*C, exposing asurface of said heated substrate to vapors of a cobalt containingcompound and simultaneously to vapors of an iron containing compound, toform a film of non-magnetic cobalt-iron oxide chemically bonded to saidsubstrate, and thereafter heating said film of non-magnetic cobalt-ironoxide to a temperature of between about 300*C and about 600*C in areducing atmosphere to reduce the material of said film to cobaltsubstituted magnetite.
 10. The method of claim 9 wherein said cobaltcontaining compound is selected from the group consisting of cobaltnitrosyltricarbonyl, cobaltocene, cobaltic acetylacetonate, cobaltousacetylacetonate, cobaltic hexafluoroacetylacetonate, and cobaltoushexafluoroacetylacetonate, and said iron containing compound is selectedfrom the group consisting of iron pentacarbonyl, ferrocene, ferricacetylacetonate, ferrous acetylacetonate, ferrichexafluoroacetylacetonate, and ferrous hexafluoroacetylacetonate. 11.The method of claim 9 wherein said cobalt containing compound is cobaltchloride, and the iron containing compound is ferric chloride.
 12. THEMETHOD OF CLAIM 9 WHEREIN SAID NON-MAGNETIC SUBSTRATE IS MADE OF ANION-EXCHAGE STRENGTHENED GLASS.